Ionic liquids are salts composed of cationic and anionic components whose structures impart a sub-room temperature melting point or glass transition to the resulting material. A liquid character is associated with ions that have very weak tendencies to coordinate toward oppositely charged ions (e.g. charge delocalized or sterically shielded), with substituents that have weak intermolecular forces (e.g. fluorocarbons, alkanes, silicones) and with a structural symmetry that is not conducive to efficient molecular packing. Most ionic liquids are organic salts. The cationic component is usually organic in nature (e.g. alkyl-substituted ammonium, phosphonium, imidazolium and pyridinium), and the anionic component is most often inorganic (e.g. nitrate, sulfate, thiocyanate, halide, tetrafluoroborate, hexafluorophosphate, etc.) but may also be organic (e.g. tosylate, alkylsulfates, fluoroalkylsulfates, alkylcarboxylates, fluoroalkylcarboxylates, etc.). These liquid materials have unique properties (immeasurably low volatility, non-flammability, very high polarity and solvating characteristics, high ionic conductivity, and a wide electrochemical potential window). There is currently much interest in their use as solvents for a large variety of reactions and in sampling for chemical analysis. See Zhao et al., J. Chem. Technol. Biotechnol. 2005, 80, 1089 and Welton, Chem. Rev. 1999, 99, 2071.
A more unique form of ionic liquid is based on a system where one of the two charged components is a polymer. As such, each repeat unit incorporates the same ionic site. There are very few reports in the literature of such systems. They are mostly based on the imidazolium ion functionalized with a polymerizable vinyl, acrylic or styryl moiety and have the physical form of a glass or sticky rubber. Reports include imidazolium polymers. See Washiro, et al. Polymer, 2004, 45, 1577; Ding, et al., J. Poly. Sci. Part A, 2004, 42, 5794; Tang, et al. J. Poly. Sci. Part A, 2005, 43, 5477; and Nakajima, et al., Polymer, 2005, 46, 11499 and alkali metal sulfonate polymers, see Ogihara, et al., Electrochim. Acta, 2004, 49, 1797 and Binnemans, Chem. Rev., 2005, 105, 4148. These polymers are prepared from monomers which themselves may or may not be ionic liquids. In the case of cationic imidazolium polymers, certain imidazolium monomer ionic liquids will yield the corresponding polymer ionic liquid if appropriate substitution is made on the imidazole ring; otherwise, a glassy solid is obtained. An appropriate substitution relates to the addition of a sufficient number and/or sufficient size of alkyl groups to the ring. In contrast, the anionic-form of a polymer ionic liquid has yet to be prepared directly from its analogous monomer ionic liquid. For instance, the high melting sulfonate monomer solid (usually an alkali metal salt) is first polymerized in solution followed by substitution with an appropriate cationic counter ion. Solvent removal is necessary to generate the anionic polymer liquid, still retaining the alkali metal ion.
Polymerizable ionic liquids and their actuation in an electric field are a combination of material and properties with unique potential to display structural and fluid dynamics above that found for small molecule ionic liquids. Small molecule ionic liquids are generally monovalent organic salts with melting points or glass transitions below room temperature. They derive their liquid character from a selection of ionic structures which have very weak tendencies to coordinate with oppositely charged ions, low intermolecular forces and low symmetry. Their properties (immeasurably low volatility, non-flammability, very high polarity and solvating characteristics, high ionic conductivity, and a wide electrochemical potential window) are of substantial interest particularly with regard to applications as green solvents, analytical extraction solvents, and electrochemical supporting media. Very recently it has been reported that water immiscible ionic liquids display significant electrowetting characteristics with an interesting dependence on the size of the cationic and anionic components. See Ralston, et al., J. Am. Chem. Soc., 2006, 126, 3098. Ionic liquids themselves provide an opportunity of producing a more stable actuating medium, eliminating such issues as solvent evaporation and degradation due to electrolysis, typically found in aqueous based electric field induced actuators.
In an ionic liquid polymer system the cationic or anionic centers are constrained to repeat units in the polymer chain. As such, any molecular flow or diffusion requires a concerted motion of as many ionic centers as there are charged repeat units in the polymer chain. When subjected to an electric field, a polymeric system may respond in an enhanced or retarded manner relative to a small molecule ionic liquid, depending on whether the covalent linkage of cationic or anionic repeat units responds as a more highly charged single molecule or whether its macromolecular size inhibits molecular motion needed for a response.